Oxidation of Fe- and Ni-aluminides in Dry and Humid Atmospheres

Doktorsavhandling, 2001

Aluminides based on Ni3Al and Fe3Al offer good mechanical and corrosion properties at high temperatures and are therefore being considered in the automotive and chemical industries. The aim of this work was to study the early stages and the mechanisms of oxide growth on unalloyed Ni3Al and Fe3Al in air. The influence of water vapour on the oxidation behaviour and the role of alloying elements, particularly Cr, were also investigated. The oxide products were mainly analysed by X-ray Photoelectron Spectroscopy, Auger Electron Spectroscopy and Scanning Electron Microscopy.
Ni3Al and Fe3Al are shown to behave differently at intermediate temperature (up to 500°C), regarding both the oxide growth rate and morphology. At room temperature and 300°C, the surface of Ni3Al is covered with islands of Al2O3 and NiO mixed with NiAl2O4. At 500°C, the Ni-oxides growing by outward diffusion of cations expand laterally and cover the entire surface. In the case of Fe3Al, the early-formed islands of Al2O3 and Fe2O3 mixed with FeAl2O4 grow to form a two-layer structure: an outer layer of Fe-oxides on top of a layer of Al2O3. During further oxidation, the continuous alumina layer is broken and islands of Fe2O3 mixed with FeAl2O4 nucleate at the surface. At 300°C, the oxide formed on Fe3Al is thicker than on Ni3Al, whereas it is the contrary at 500°C.
The oxides formed on the aluminides at 500°C in dry oxygen have the same composition and morphology as in air. The addition of water vapour does not significantly influence the oxidation of Ni3Al. However, it causes a breakdown of the protecting properties of the alumina layer on Fe3Al, resulting in an increased oxide growth rate. Additions of 2 and 4 at.% Cr are detrimental to the oxidation resistance of Ni3Al and Fe3Al at 500°C in dry air because of the growth of oxide particles at the surface. Among the different alloying elements added to Ni3Al oxidised at 900°C in 1 atm oxygen, Cr (6 at.%) together with Mo (4 at.%) show the best results. Adhesion of the oxide scale is improved by addition Zr, but high concentrations of Zr (1 at.%) lead to internal oxidation by precipitation of ZrO2.